Current limiting fuse

ABSTRACT

A current limiting fuse structure comprising a generally tubular, electrically insulating casing having terminal means disposed adjacent to the opposite ends thereof. One or more fusible elements are connected between the terminal means with each fusible element including first and second conductor sections of different sizes or cross-sectional areas. A pulverulent, arc quenching material is also disposed in the casing in contact with each fusible element.

United States Patent [72] Inventor Frank L. Cameron Irwin, Pa. [21] Appl. No. 872,895 [22] Filed 0ct.31, 1969 [45] Patented Mar. 9, 1971 [73] Assignee Westinghouse Electric Corporation Pittsburgh, Pa.

[54] CURRENT LIMITING FUSE 6 Claims, 4 Drawing Figs. [52] US. Cl 337/290, 337/161, 337/293 [51] Int. Cl H0111 85/08, H01h 85/12, H01h 85/14 [50] Field ofSearch 337/160, 161, 162, 290, 293, 295, 296

[56] References Cited UNITED STATES PATENTS 2,392,703 l/l946 Schurig 337/162 2,157,906 5/1939 Lohausen 337/295 1,626,105 4/1927 Sunot 337/290 FOREIGN PATENTS 556,618 10/1943 Great Britain 337/160 Primary Examiner-G. Harris Assistant Examiner-Dewitt M. Morgan Attorneys-A. T. Stratton and C. L. McI-Iale ABSTRACT: A current limiting fuse structure comprising a generally tubular, electrically insulating casing having terminal means disposed adjacent to the opposite ends thereof.

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Patented March 9, 1971 INVENTOR Frank L. Cameron ATTORNEY CURRENT LIMITING FUSE BACKGROUND OF THE INVENTION In the construction of current limiting fuses, particularly those of the high voltage type, it has been found desirable or necessary to provide fusible elements or fuse links of stepped size in order that the overall fuse be capable of interrupting and limiting overload currents of both relatively large and small values within a predetermined range of overload currents. Such known fuse structures of the above general type are illustrated in US. Pat. Nos. 2,090,609; 2,094,012; 2,157,906; 2,157,907; 2,162,540; 2,199,746 and 3,213,242. In such known fuse structures, the different size conductors of each fusible element are normally disposed in end to end relation and joined in butt or lap joints by suitable means, such as welding or brazing. When a current limiting fuse of, the above general type is required for relatively low current ratings, the conductor sections which make up each fusible element are of such relatively small sizes or cross-sectional areas that problems arise in joining the ends of the different conductor sections which form the fusible element by welding or brazing.

SUMMARY OF THE INVENTION In accordance with the invention, a current limiting fuse structure comprises a generally tubular electrically: insulating casing or housing having terminal means mounted on said casing adjacent to each of the opposite ends of the casing. One or more fusible elements is disposed in the casing and connected between the associated terminal means. Each of the fusible elements includes first and second conductor sections of different cross-sectional areas or sizes. Each of the conductor sections of one of the fusible elements is bent back on itself to form a central loop or bight portion and a pair of conductor portions which extend away from the associated loop portion. The loop portions of the conductor sections of each fusible element are assembled or disposed to pass through each other intermediate the ends of the fusible element and with the associated pairs of conductor portions of said conductor sections extending away from the associated loop portions in opposite directions toward the respective terminal means. In an illustrated embodiment, each fusible element is helically wound or disposed on an electrically insulating support member which is disposed in the casing and which may be gas evolving to aid or assist in art extinction or interruption and may also have antitracking characteristics. In addition, in an illustrated embodiment of the invention an M effect causing means, more specifically a quantity of a low melting point, metallic alloy, such as tin-lead, may be disposed on each fusible element adjacent to or on the loop portions of the conductor sections which make up each fusible element to insure an interrupting operation of the current limiting fuse at relatively low over load currents. In an illustrated embodiment of the invention, a quantity of finely divided, pulverulent or granular arc quenching material is disposed in the casing of the fuse structure in contact with each fusible element.

It is therefore an object of the invention to provide an improved current limiting fuse construction.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects of the invention will be apparent from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 11 is a view, partly in elevation and partly in section, of a current limiting fuse structure embodying the invention;

FIG. 2 is a view in section of the fuse structure shown in FIG. 1, taken along the line 11-11 of FIG. 1;

FIG. 3 is an elevational view of an electrically insulating support member which forms part of the fuse structure shown in FIG. I and on which the fusible elements of the overall fuse structure are disposed; and

FIG. 4 is an enlarged, isometric view of one of the fusible elements which forms part of the fuse structure shown in FIGS. 1 through 3, illustrating the manner in which the fusible element is formed.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings and FIGS. 1 and 2 in particular, there is illustrated a current limiting fuse structure 10 which is particularly adapted for high voltage applications, such as 5 KV and above, and which embodies the principal features of the invention. As illustrated, the fuse structure 10 includes a generally tubular casing or housing 20 which is formed from a suitable electrically insulating material which has sufficient structural strength to withstand the thermal conditions and internal pressures which may result during the operation of the fuse structure 10, such as a glass-reinforced melamine resin material. In order to close off the opposite ends of the casing 20 and to provide means for making electrical connections to the fuse structure 10 adjacent to the ends thereof, the terminal end caps or electrically conducting ferrules 62 and 64 are secured to the opposite ends of the casing 20 by suitable means, such as the magnetic forming method which is described in detail in US. Pat. No. 3,333,336 which issued Aug. 1, l967to F. L. Cameron and W. C. Good and which is assigned to the same assignee as the present applica-' tion. Where desired, the axially projecting electrically conducting studs 72 and 74 may be-mounted on or integrally formed with the respective terminal end caps 62 and 64, respectively, in order to permit the mounting of the fuse structure 10 in particular types of supporting structures.

In order to assist in supporting the fusible elements 40, which will be described in detail hereinafter, to assist in properly positioning the fusible elements 40 inside the casing 20 at a location which is spaced from the inner bore of the easing 20 and for other purposes where desired, the axially extending electrically insulating support member 30 is disposed inside the casing 20. As illustrated in 'FIG. 2, the support member 30 is generally star-shaped in configuration and includes a plurality of circumferentially spaced, outwardly projecting wall portions 36. The support member 30 is preferably formed or molded from an electrically insulating material which has sufficient structural strength to withstand the thermal conditions and pressures which result during an interrupt ing operation of the fuse structure 10 and may be a glass-reinforced thermosetting resin, such as a polyester resin. The material from which the support member 30 is formed may also include a filler material, such as aluminum trihydrate, to provide the support member 30 with antitracking characteristics. Where desired, the material selected for the support member 30 may also be of the gas evolving type such as a glass-reinforced polyester material to assist in arc interruption during the operation of the fuse structure 10. Where the support member 30 is formed from a material which is also gas evolving, the support member 30 may include a plurality of axially spaced recesses as indicated at 34 in each of the outwardly projecting wall portions 36 in order to prevent the formation of an electrically conducting path along the different surfaces of the support member 30 during an interrupting operation of the fuse structure 10. In order to better support and retain the fusible elements 40 in the assembled positions, as indicated in FIG. 3, the support member 30 may include a plurality of axially spaced groups of recesses or cuts as indicated at 38 on each of the outwardly projecting wall portions 36 of the support member 30.

In general, the fuse structure 10 includes one or more fusible elements 40 which are electrically connected and extend between the terminal end caps or terminal members 62 and 64. As illustrated, the fuse structure 10 includes two fusible elements 40 which are electrically connected in parallel between the terminal members 62 and 64. More specifically, as shown in FIG. 4, each of the fusible elements 40 comprises first and second electrical conductor sections 42 and 44, respectively, which are formed from electrical conductors, such as wire conductors, having different cross-sectional areas or sizes. For example, the first conductor section 42 may be formed from a relatively smaller diameter wire conductor and the second conductor section 44 may be formed from a relatively larger diameter wire conductor. As shown in FIG. 4,

each of the conductor sections 42 and 44 includes a predetermined length of electrical conductor, such as silver, which is bent or curved back on itself to form the loop portions indicated at 42A and 44A, respectively, which pass through each other and contact one another intennediate the ends of the overall fusible element 40. The conductor sections 42 and 44 include the pairs of conducting portions 42B and 42C and 44B and 44C, respectively, which project or extend away from the associated loop portions 42A and 44A, respectively. The pair of conductor portions 42B and 42C of the conductor section 42 and the pair of conductor portions 448 and 44C of the conductor section 44 project away from the associated loop portions 42A and 44A in opposite directions toward the respective terminal members 62 and 64, respectively shown in FIGS. 3 and 4. As also shown in FIG. 4, each fusible element 40 includes a ball or bead of M effect causing material or means as indicated at 46 which comprises a quantity of low melting point metallic alloy, such as tin-lead. The effect of the low melting point ball 46 is to modify the low current melting characteristics of the fusible element 40 and to improve the ability of the fuse structure to interrupt relatively low overload currents at high voltages. It is to be noted that the number of fusible elements 40 required in a particular application depends on the current rating of the fuse structure 10 with additional fusible elements 40 being electrically connected in parallel to provide the necessary current rating of the overall fuse structure 10. It is to be noted that the loop portions 42A and 44A of the conductor sections 42 and 44 respectively preferably contact or engage one another directly rather than relying on the ball of low melting point material 46 to provide the only electrically conducting path between the conductor sections 42 and 44 in order to limit the effective resistance of the fusible element 40 which might otherwise develop during the operation or life of the overall fuse structure 10.

As best shown in FIGS. 2 and 3, the fusible elements 40 may be helically wound or disposed on the associated supporting member 30 and, more specifically on the circumferentially spaced wall portions 36 of the supporting member 30 with the turns of the fusible elements 40 being retained in the assembled positions as shown in FIG. 3 by the notches or recesses 38 provided at the outer peripheries of the respective wall portions 36. In order to provide the necessary electrical connections between the opposite ends of the fusible elements 40 and the associated terminal members 62 and 64, the ends of the fusible elements 40 are collected at each end of the overall fuse structure 10 and twisted together to form the conducting loops 82 and 84 shown in FIG. 3. The conducting loops 82 and 84 are then looped over the opposite ends of the casing prior to the assembly and securing of the terminal members 62 and 64 on the casing 20 so that when the terminal members 62 and 64 are secured to the ends of the casing 20 by any suitable means, such as the magnetic forming previously mentioned, the ends of the fusible elements 40 will be securely fastened to the associated terminal members 62 and 64 and provide the necessary electrical connections between the fusible elements 40 and the associated terminal members 62 and 64. It is to be noted that the ends of the fusible elements 40 may pass through openings provided in the support member as indicated at 32 in FIG. 3.

In order to additionally aid in arc extinction or interruption during the operation of the fuse structure 10 and to provide the current limiting action which is necessary in a fuse structure of the type described, the space between the casing 20 and the fusible elements and between the casing 20 and the supporting member 30 is substantially filled with a finely divided, pulverulent or granular arc-quenching material such as silica sand or quartz sand, in which the fusible elements 40 are effectively embedded. It is to be noted that after the casing is substantially filled with the arc-quenching material which is indicated at 52in FIGS. 1 and 2, the arc-quenching material 52 may then be compacted by any suitable means, such as vibration or other known methods.

In the operation of the overall fuse structure 10, when an abnormal or overload current starts to flow through the fusible elements 40 of the fuse structure 10, the conductor section 42 having a relatively smaller cross-sectional area will begin to melt initially and a series of arc voltages will develop between the particles or drops of vaporized or melted conductor material of the conductor portions which make up the conductor section 42. As the melting of the conductor section 42 proceeds, the arc voltage which develops will increase quickly to a peak value prior to the melting of the other conductor section 44. As the size of the arc current which flows due to the melting of the conductor section 42 increases, the peak arc voltage which develops will tend to partially collapse due to the increased size of the arc current which develops due to the melting of the conductor section 42. When the melting of the conductor section 42 is substantially completed, the conductor section 44 having a relatively larger cross-sectional area will then begin to melt or burn back and to effectively increase the arc voltage which is produced by the sequential melting of the different conductor sections of the fusible element 40. In other words, the stepped construction of the fusible elements 40 will not result in a single arc voltage peak during the operation of the fuse structure I0 but will tend to sustain the arc voltage for a sufficient period of time to insure the interruption of the arc current which results during the operation of the fuse structure 10. The construction of the fusible elements 40 permits a relatively shorter longitudinal dimension of the overall fuse structure 10 than if fusible elements were employed having only a single cross-sectional area and therefore results in a more compact construction of the fuse structure 10. It is to be noted that during the operation of the fuse structure 10, the arc-quenching material 52 which is disposed in the casing 10 will aid in arc interruption by absorbing the thermal energy of the arc currents which develop during the operation of the fuse structure 10 and form a fulgurite with the vaporized material of the fusible elements 40 as is well known in the fuse art. In addition, where the support member 30 is formed from a gas evolving material, the gases evolved from the support member 30 when subjected or exposed to arcs will also assist in the interruption of the overload currents which the fuse structure 10 is intended to interrupt. The portion of the support member 30 which is exposed to arcs during an interrupting operation of the fuse structure 10 is limited due to the limited contact between the fusible elements 40 and the wall portions 36 ofthe support member 30.

The current limiting fuse structure embodying the teachings of this invention has several advantages. For example, a current limiting fuse including fusible elements as disclosed of the stepped type is relatively more compact because of the relatively sustained arc voltages which result during the operation of the disclosed fuse structure. In addition, the disclosed fuse structure is uniquely adapted to provide a fuse structure having fusible elements of the stepped type for relatively low current ratings where problems will arise in joining electrical conductors of different cross-sectional areas and relatively small sizes in an overall fusible element. A further advantage of the applicant's invention is that the fuse construction disclosed more readily lends itself to a current limiting fuse structure which is capable of interrupting currents over a full range of overload currents. Finally, a fuse structure including fusible elements as disclosed lends itself readily to the disposition of a low melting point metallic alloy intermediate the ends of the fusible elements as disclosed.

It is to be understood that in certain applications a fuse structure as disclosed may include only a single fusible element and that in such a fuse structure a separate support means such as the support member 30 may be omitted where the fusible element is sufficiently rigid as to not require a separate support member.

Since numerous changes may be made in the above described apparatus and different embodiments of the invention may be made without departing from the spirit and scope thereof, it is intended that all matter contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

lclaim:

1. A fuse structure comprising a generally tubular insulating casing, terminal means disposed adjacent .to each of the 0pposite ends of said casing, a fusible element connected between said terminal means, said fusible element comprising first and second conductor sections having different cross-sectional areas each bent back on itself to form a central loop portion and a pair of conductor portions extending away from the associated loop portion, the loop portions of said fusible element passing through each other intermediate the ends of said fusible element and the pairs of conductor portions of said conductor sections of said fusible element extending away from the associated loop portions in opposite directions.

2. The combination as claimed in claim'-l wherein an M effect causing means is disposed on said fusible element adjacent to the loop portions of said conductor sections.

3. The combination as claimed in claim 1 wherein a quantity of pulverulent, arc quenching material is disposed in said casing in contact with said fusible element.

4. The combination as claimed in claim 1 wherein an axially extending electrically insulating support member is disposed in said casing and said fusible element is helically disposed on 

1. A fuse structure comprising a generally tubular insulating casing, terminal means disposed adjacent to each of the opposite ends of said casing, a fusible element connected between said terminal means, said fusible element comprising first and second conductor sections having different cross-sectional areas each bent back on itself to form a central loop portion and a pair of conductor portions extending away from the associated loop portion, the loop portions of said fusible element passing through each other intermediate the ends of said fusible element and the pairs of conductor portions of said conductor sections of said fusible element extending away from the associated loop portions in opposite directions.
 2. The combination as claimed in claim 1 wherein an ''''M'''' effect causing means is disposed on said fusible element adjacent to the loop portions of said conductor sections.
 3. The combination as claimed in claim 1 wherein a quantity of pulverulent, arc quenching material is disposed in said casing in contact with said fusible element.
 4. The combination as claimed in claim 1 wherein an axially extending electrically insulating support member is disposed in said casing and said fusible element is helically disposed on said support member.
 5. The combination as claimed in claim 4 wherein a pulverulent, arc quenching filler is disposed in said casing in contact with said fusible element.
 6. The combination as claimed in claim 5 wherein an ''''M'''' effect causing means is disposed on said fusible element intermediate the ends thereof adjacent to the loop portions of said conductor sections of said fusible element. 